This invention pertains to an apparatus and method for point-to point communicating over electrical power line cables. More particularly, this invention pertains to an apparatus and a method of communicating by inductive coupling to a single power-line lead that includes two parallel conductors.
Electrical equipment requires power lines for supplying either direct current or alternating current. Power line connections, or power leads, to electrical equipment generally include two or more power leads and may include a ground lead. For direct current power equipment, the two power leads are the positive and negative leads and optionally, a ground lead. Single phase alternating current equipment power leads include a phase lead, a neutral lead and optionally, a ground lead. For multi-phase power equipment, the power leads include a lead for each phase, and optionally, a neutral lead and/or a ground lead.
In industrial settings, it is often a requirement to control or operate motors locally, that is, at the motor location. Also, indication of equipment status or other information is often required locally. However, the controller for the electrical equipment, such as the equipment that switches the power to the motor, is typically located remotely, that is, at a location some distance away from the motor location. Communication of control and indication signals between motor drive controllers and the driven motor has traditionally been done through additional cables and wiring separate from the power leads. The use of dedicated control and indication wiring increases the installation cost of motor driven systems. Power line communication systems have been developed using capacitive coupling for connecting to the power lines. However, these systems have electrical isolation and noise issues.
According to one embodiment of the present invention, an apparatus and method for a power line communications system is provided. The power line communications system does not require dedicated communications or control wiring; instead the system utilizes the electrical power leads connecting two electrical devices.
The communications signal in this system is inductively coupled to one of the power leads. The one power lead is wired with two parallel conductors; that is, the single lead is split into two similarly sized conductors, each carrying approximately half the load current of that power lead. The two parallel conductors are coupled to a plurality of current transformers; each current transformer is located at a point where communications are desired to be received or transmitted. Each of the two conductors is passed, in opposite directions, through the center opening of each current transformer. That is, the direction of current flowing in one of the two conductors is opposite that of the other of the two conductors, relative to the current transformer. Because the similarly sized conductors are carrying the same load current, the current transformers see a net load current of zero.
The two conductors form a loop and serve as the primary winding for the current transformer. The communications signal is applied to the secondary winding of one current transformer, which induces a signal on the primary winding. The induced signal from the transmitting current transformer is sensed by the receiving current transformer.
The method for power line communications includes the steps of forming a loop from one of the power leads connecting two pieces of electrical equipment, inductively inducing a communications signal on that loop at a first point, and sensing the induced signal on the loop at a second point. Additionally, the method includes the steps of converting control and other signals into a communications signal and converting the communications signal into control and other signals.